NO177192B - Synthetic two-component fiber and process for its preparation - Google Patents

Abstract

A thermobondable bicomponent synthetic fibre with a length of at least about 3 mm, adapted to use in the blending of fluff pulp for the production of hygiene absorbent products, the fibre comprising an inner core component and an outer sheath component, in which the core component comprises a polyolefin or a polyester, the sheath component comprises a polyolefin, and the core component has a higher melting point than the sheath component, and a process for producing said fibre. The sheath-and-core type fibre is preferably made permanently substantially hydrophilic by incorporating a surface active agent into the sheath component. The long bicomponent fibres form a strong supporting three-dimensional matrix structure in the absorbent product upon thermobonding.

Description

The present invention relates to a thermo-bondable, hydrophilic synthetic fiber consisting of two components (bicomponent synthetic fiber, hereinafter called synthetic two-component fiber) for use when mixing down-like mass, and a method for producing the fiber. More specifically, the invention relates to a fiber comprising an outer covering component (sheath component) and an inner core component (core component), the core component having a higher melting point than the covering component. The fiber is permanently largely hydrophilic. The term "hydrophilic" refers to the fact that the fiber has an affinity for water and is thus easily dispersed in water or aqueous mixtures. This affinity can be attributed to the presence of polar groups on the fiber's surface. The term "permanent" substantially hydrophilic refers to the fact that the fiber will retain its hydrophilic properties after repeated dispersions in water. This is achieved by incorporating a surface-active agent and possibly a hydrophilic polymer or copolymer in the fiber's coating component. The fiber according to the invention is useful for the production of "down" ("fluff"), which is a downy fibrous material used as an absorbent material, and/or liquid-conducting core in the production of absorbent hygienic products such as disposable nappies. Down (fluff) is produced by fibering and dry forming of so-called "fluff pulp" which consists of natural and/or synthetic fibres.

In recent years, there has been a trend towards stronger, thinner and lighter disposable nappies and other hygienic absorbent disposable products. One factor in this direction has been the development of a number of synthetic fibers, particularly heat-adhesive (thermosetting) synthetic fibers that have been used to replace at least some of the natural cellulose fibers in these products. Such thermosetting synthetic fibers are typically used to bind together the cellulose fibers to thereby obtain an absorbent material with improved strength and allow the production of thinner and lighter products. Examples of patents describing such fibers or their use or production are US-PS 4,189,338 (Nonwoven fabric comprising side-by-side bicomponent fibers), 4,234,655 (Heat adhesive composite fibers), 4,269,888 (Heat adhesive composite fibers) , 4,425,126 (fibrous material using thermoplastic synthetic fibers), 4,458,042 (absorbent material containing polyalkene pulp treated with a wetting agent) and 4,655,877 (absorbent continuous (web) structure containing short hydrophilic thermoplastic fibers) and EP Patent Application No. 0 248 598 (non-woven fabric of polyalkene type).

However, the use of these synthetic fibers in absorbent products has not been without problems. One problem that can be encountered is that it can be difficult to distribute the synthetic fibers in the fluffy mass produced by a wet process, as these synthetic fibers are generally of a hydrophobic nature. Such hydrophobic fibers repel water and therefore tend to form clumps in the fluffy mass or to float on the surface of the wet fluffy mass if they are lighter than water. If the synthetic fibers are also unevenly distributed in the down, barriers that prevent the transport of moisture can be formed in the absorbent product due to fusion of the thermoset fibers to each other in areas where there is an accumulation of such fibers. Furthermore, the synthetic fibers that are currently used in the production of down are generally quite short, i.e. normally shorter than the cellulose fibers that typically make up a significant part of the down. The supporting structure of the absorbent material is therefore formed by the cellulose fibers in the material, and as absorbent cores of such natural cellulose fibers tend to break and bend under the loads that e.g. diapers are exposed to, wicking barriers are easily formed. Absorbent cores consisting only of natural cellulosic fibres, i.e. containing no synthetic fibres, can likewise be subject to breakage and the formation of absorbent barriers due to stresses and bending.

Hygienic absorbent products often include a so-called super absorbent polymer in the form of a powder or small particles, which is incorporated into the material to achieve a weight saving. However, the superabsorbent polymer in these materials often tends to move out of the position where it was originally placed due to the lack of a structure that can effectively retain the small particles.

The long two-component synthetic fiber according to the invention is aimed at the problems mentioned above. The two-component fibers according to the invention are significantly longer than other fibers that are typically used in the production of down. During the production of absorbent products from down containing the two-component fiber, the down is subjected to a heat treatment (thermosetting) where the cover component of the two-component fiber is melted, while the high-melting core component of the fiber remains intact. The core component in the long two-component fibers is thus fused together by the melting of the covering component to form a strong, uniform, supporting, three-dimensional base mass or matrix in the absorbent material. The absorbent material is thus able to resist bending without developing absorption barriers due to breakage of the absorbent core. In addition, the matrix structure formed by the two-component fibers gives the material an improved ability to retain its shape under dynamic stress when using the absorbent product.

The three-dimensional net-like structure formed by the high-melting component of the two-component fiber in the thermoset material allows the superabsorbent polymer to be held in the desired position. This is a further advantage which provides more efficient use of the superabsorbent polymer and helps to increase the porosity as well as making the production of lighter absorbent materials possible.

In addition, the low-melting coating component has been made permanently largely hydrophilic, which allows the fibers to be distributed homogeneously in the wet-treated downy mass, which is typically used in the manufacture of absorbent material. It is also desirable that the fibers in the finished product are hydrophilic, so that the product's absorbent and liquid-conducting properties are not impaired, which may be the case in a product with a significant content of hydrophobic fibres.

The present invention relates to a thermosetting, hydrophilic synthetic two-component fiber for use in mixing fluff pulp, comprising an inner core component and an outer covering component, where

- the core component comprises a polyalkene or a polyester,

- the coating component comprises a polyalkene, and

- the core component has a higher melting point than the coating component,

as the fiber is permanently largely hydrophilic due to the incorporation in the coating component of from 0.1-5%, calculated on the total weight of the fiber, of a surface-active agent, e.g. a fatty acid ester of glycerol, a fatty acid amide, a polyglycol ester, a polyethoxylated amide, another non-ionic surfactant, a cationic surfactant or a mixture of the above and/or other compounds normally used as emulsifiers, surfactants or detergents, and the fiber has a length of 3-24 mm.

In a bicomponent fiber of the cladding-and-core type, the core component is surrounded by the cladding component, in contrast to a bicomponent fiber of the side-by-side or bilateral type, where the two components both have a continuous longitudinal outer surface. However, a small portion of the core component may be exposed at the surface in the case of so-called "acentric" cover-and-core fiber as explained below.

The covering component in the two-component fiber is selected from the group of polyalkenes, while the core component may comprise a polyalkene or a polyester. The core component typically has a melting point of at least 150°C, preferably at least 160°C and the coating component typically has a melting point of 140°C or lower, preferably 135°C or lower. The two components of the fiber thus have melting points that are significantly different from each other, which allows the low-melting coating component to be melted in a thermosetting process, while the high-melting core component remains largely intact. Although specific melting points are indicated below, it must be remembered that these materials, like all crystalline polymeric materials, actually melt gradually over a range of a few degrees. However, this is not a problem, because in practice the two components of the fiber will be chosen so that their melting points are significantly different from each other.

Preferably, the fiber includes a coating component comprising a low-melting polyalkene such as a high-density polyethylene (melting point (m.p.) about 130°C), low-density polyethylene (m.p. about 110°C), linear low-density polyethylene (m.p. about 125°C) or poly(1-butene) (m.p. about 130°C) or mixtures or copolymers thereof, together with a core component comprising a polyalkene such as polypropylene (m.p. about 160°C). The coating component can further comprise an ethylene-propylene copolymer based on propylene with up to approx. 7% ethylene (m.p. approx. 145°C).

The fiber according to the invention can also include a core component comprising poly(4-methyl-l-pentene) (m.p. approx. 230°C) and a coating component comprising any of the above-mentioned polyalkenes, (i.e. high density poly -ethylene, low density polyethylene, linear low density polyethylene, poly(1-butene) or polypropylene).

Alternatively, the core component may comprise a polyester with a high melting point (ie above about 210°C) such as poly(ethylene terephthalate) (m.p. about 255°C), poly(butylene terephthalate) (m.p. about 230° C) or poly(1,4-cyclohexylene-dimethyl-terephthalate)

(m.p. approx. 290°C) or other polyesters or copolyesters which comprise the structures stated above and/or other polyesters. If the fiber includes a polyester core, the coating may comprise any of the materials mentioned previously, i.e. high density polyethylene, low density polyethylene, linear low density polyethylene, poly(1-butene),

polypropylene or copolymers or mixtures of these materials), or another material with a melting point of approx. 170°C or lower.

In addition, the coating component may comprise a mixture of e.g. low density polyethylene and either an (ethyl vinyl acetate) copolymer or an (ethylene acrylic acid) copolymer (m.p. about 100°C) as explained below.

The mixture of the fiber's two components can thus be varied to include a number of different base materials, and the exact composition in each case will obviously depend on the material in which the fiber is to be used, as well as the equipment and the production processes used to manufacture the absorbent in question material.

The fiber has been given permanent hydrophilic surface properties by incorporating a surfactant into the coating component and optionally by incorporating a hydrophilic polymer or copolymer into the coating component.

The surface-active agent can typically be selected from compounds that are normally used as emulsifiers, surface-active agents or detergents, and can comprise mixtures of these compounds. Examples of such compounds are fatty acid esters of glycerol, fatty acid amides, polyglycol esters, polyethoxylated amides, non-ionic surfactants and cationic surfactants.

Specific examples of such compounds are a polyethylene glycol lauryl ether having the formula:

glycerol monostearate having the formula: erucamide having the formula: stearic acid amide having the formula: a trialkyl phosphate having the formula: alkyl phosphate amine ester having the formula: a lauryl phosphate potassium salt having the formula: or: and an ethylenediamine polyethylene glycol having the formula:

The compounds should preferably have a hydrophobic part to make

them compatible with the alkenic polymer and a hydrophilic part to make the surface of the fiber wettable. Mixtures of compounds can be used to regulate the hydrophilic properties. The surfactant is typically incorporated into the coating component in an amount of 0.5-2%, calculated on the total weight of the fiber. This amount of surfactant is sufficient to give the fiber the desired hydrophilicity, without

it has some adverse effects on other properties of the fiber.

The coating component may also comprise a hydrophilic polymer or hydrophilic copolymer. Examples of such a hydrophilic copolymer are (ethyl vinyl acetate) copolymer and (ethylene acrylic acid) copolymer. In this case, the coating component may comprise, in addition to the surfactant as described above, a mixture of e.g. 50-75% low density polyethylene and 50-25% of the hydrophilic copolymer, and the amount of vinyl acetate or acrylic acid will respectively be typically 0.1-5%, and preferably 0.5-2%, calculated on the total weight of the fiber.

The fibers can be tested for hydrophilicity by e.g. measuring the time required for them to sink in water, e.g. according to the European Disposable Non-woven Association standard no. 10.1-72. The fibers can be placed in a metal mesh on the surface of the water, and they can be defined as hydrophilic if they sink below the surface within approx. 10 s and preferably within approx. 5 p.

The weight ratio of the cover and core components in the two-component fiber is preferably in the range from 10:90 to 90:10. If the cover component includes less than approx. 10% of the total weight of the fiber, it may be difficult to achieve sufficient thermosetting of the core component to other fibers in the material. Likewise, if the core component comprises less than approx. 10% of the total weight of the fiber, it may not be possible for the thermoset core component to impart sufficient strength to the finished product. More precisely, the weight ratio between cover component and core component will typically lie in the range from 30:70 to 70:30 and preferably from 40:60 to 65:35.

The cross-section of the two-component fiber is preferably circular, as the equipment typically used for the production of synthetic two-component fibers normally produces fibers with a largely circular cross-section. However, the cross-section can also be oval or irregular. The design of the cladding and core components can be either concentric or acentric (as shown in Fig. 1), the latter design or configuration being sometimes known as a "modified side-by-side" or an "eccentric" two-component fiber. The concentric configuration is characterized by the covering component having a largely uniform thickness so that the core component lies approximately in the center of the fibre. In the acentric configuration, the thickness of the cladding component varies, and the core component is therefore not in the center of the fiber. In both cases, the core component is largely surrounded by the cover component. In an acentric two-component fibre, however, part of the core component can be exposed, so that in practice up to approx. 20% of the surface of the fiber is made up of the core component. The coating component in a fiber with an acentric design will nevertheless comprise the main part of the fiber's surface, i.e. at least approx. 80%. Both the cross-section of the fiber and the design of the components will depend on the equipment used to produce the fiber, the process conditions and the molecular weight of the two components.

The fibers preferably have a fineness of 1-7 decitex (dtex), a decitex is the weight in grams of 10 km of fibre. The length of the fibers must be taken into account when choosing the fineness of such fibers, and because, as explained below, the two-component fibers according to the invention are relatively long, the fineness should be adjusted accordingly. The fibers will thus typically have a fineness of 1.5-5 dtex, preferably 1.7-3.3 dtex and preferably 1.7-2.2 dtex. When more than one type of such fibers are used in the same fluffy material, e.g. fibers of different lengths, the dtex/length ratio of the individual types of fibers can be constant or variable.

The fibers are preferably crimped, i.e. given a wave shape, to make them easier to process when producing the fluffy mass. Typically, they will have from 0 to 10 ripples per cm, and preferably from 0 to 4 ripples per cm.

The length of the synthetic two-component fibers according to the invention is important, as they are significantly longer than other fibers that are typically used in the production of downy mass (fluff). For example, natural cellulosic pulp fibers that are typically the major component of fluff are normally no more than approx. 3 mm long. The heat-set synthetic fibers that are currently used in the production of fluff are typically shorter than cellulose fibers, and the cellulose fibers therefore form the basic structure of the material. The synthetic two-component fibers according to the invention are, however, significantly longer than e.g. cellulose fibers. Accordingly, the high-melting core component of the two-component fibers constitutes the basic structure of the thermoset absorbent material, which gives it improved properties with regard to strength and dimensional stability.

The fibers in the present invention are thus typically cut to a length of 5-20 mm, preferably 6-18 mm. Particularly preferred lengths are approx. 6 mm and 12 mm. The desired length is chosen according to the equipment to be used in the production of the absorbent material as well as the nature of the material itself. Although they are relatively long, the fibers are nonetheless able to pass largely intact through the mesh holes in the hammer mills used to produce fluff, as these holes typically have a diameter of 10-18 mm, as will be described below.

The fibers can be produced using a process which includes the following steps: - melting of the constituents of the core and coating components, - incorporation of from 0.1-5%, calculated on the total weight of the fibre, of a surface-active agent, e.g. a fatty acid ester of glycerol, a fatty acid amide, a polyglycol ester, a polyethoxylated amide, a non-ionic surfactant, a cationic surfactant or a mixture of the above and/or other compounds normally used as emulsifiers, surfactants or

detergents in the coating component,

- spinning of the low-melting coating component and the high-melting core component into a spun bundle of

bicomponent filaments,

- stretching of the bundle of filaments,

- preferably curling of the fibers,

- drying and fixing the fibres, and

- cutting the fibers to a length of 3-24 mm.

The above-mentioned steps shall be described in more detail as follows: The components of the coating component and the core component, respectively, are melted in separate extruders (one extruder for each of the two components), which mix the respective components so that the melts have a uniform consistency and temperature before spinning . The temperature of the molten components in the extruders is well above their respective melting points, typically more than approx. 90°C above the melting points, thus ensuring that the melts have flow properties suitable for the subsequent spinning of the fibres.

To the molten coating component is added a surfactant in an appropriate amount based on the total weight of the spun fibers, as explained above. Also, as explained above, the coating component may include a hydrophilic polymer or copolymer. The surface-active agent and the hydrophilic polymer or copolymer that may be used are important for the production of wet-processed downy pulp, because, as explained above, it is necessary that the surface of the synthetic two-component fibers be made largely hydrophilic, so that they can be distributed homogeneously in the fluffy mass. It is possible to treat the surface of the spun fibers with a wetting agent, but the result is not necessarily permanent, and thus there may be a risk that the desired hydrophilic surface properties are lost during the production of the absorbent material. By incorporating the surface-active agent and optionally the hydrophilic polymer or copolymer in the coating component before spinning, the spun fiber is made permanently largely hydrophilic, which ensures that the desired homogeneous distribution of the two-component fibers in the fluffy mass can be achieved, and that the functioning of the absorbent product will not be hindered by the presence of hydrophobic fibres.

The molten components are typically filtered before spinning, e.g. using a metal mesh to remove any unmelted or cross-linked substances that may be present. The spinning of the fibers is typically achieved by using ordinary melt spinning (also known as "long spinning"), especially ordinary spinning at medium speed, but so-called "short spinning" or "compact spinning" can also be used (Ahmed, M., Polypropylene Fibers - Science and Technology, 1982). Normal spinning comprises a two-stage process, where the first stage is the extrusion of the melts and the actual spinning of the fibers, while the second stage is the stretching of the spun fibers as they are after spinning ("as-spun"). Card spinning is a one-step process, where the fibers are both spun and stretched in a single operation.

The molten coating and core components obtained as indicated above are passed from their respective extruders through a distribution system and passed through the holes of a spinning die. The production of two-component fibers is more complicated than the production of one-component fibers, because the two components must be correctly distributed to the holes. Accordingly, in the case of two-component fibers, a special type of spinning nozzle is used to distribute the respective components, e.g. a spinning nozzle based on the principles described in US-PS 3 584 339. The diameter of the holes in the spinning nozzle is typically 0.4-1.2 mm, depending on the fineness of the fibers produced. The extruded melts are then passed through a quench channel where they are cooled by a stream of air and at the same time drawn into two-component filaments which are collected into bundles of filaments. The bundles typically contain at least approx. 100 filaments and more typically at least approx. 700 filaments. The spinning speed after the cooling channel is typically at least 200 m/min, and more typically 500-2000 m/min.

The bundles of filaments are then stretched, preferably using so-called off-line stretching or drawing which, as indicated above, takes place separately from the spinning process. Stretching is typically achieved using a series of hot rollers and a hot air furnace, where a number of bundles of filaments are stretched simultaneously. The bundles of filaments first pass through one set of rollers, followed by passage through a hot air furnace and then passage through another set of rollers. The hot rollers typically have a temperature of 70-130°C, and the hot air oven typically has a temperature of 80-140°C. The speed of the second set of rollers is greater than the speed of the first set and the heated bundle of filaments is therefore stretched according to the ratio between the two speeds (called the stretch ratio or draw ratio). Another oven and a third set of rollers can also be used (two-stage stretching), the third set of rollers having a higher speed than the second set. In this case, the stretch ratio is the ratio between the speed of the last and the first set of rollers. In a similar way, further sets of rollers and ovens can be used. The fibers according to the invention are typically stretched with a draw ratio of from 2.5:1 to 4.5:1, and preferably from 3.0:1 to 4.0:1, which leads to a suitable fineness, i.e. 1-7 dtex , typically 1.5-5 dtex, preferably 1.7-3.3 dtex, and preferably 1.7-2.2 dtex, as explained above.

The fibers are preferably crimped, typically in a so-called stuffing box (stuffer box) to make them easier to process into the fluffy pulp due to the higher fibre-to-fibre friction. The bundles of filaments are guided by a pair of pressure rollers into the chamber of the pulp box where they are crimped due to the pressure resulting from not being pulled forward inside the chamber. The degree of ripple can be regulated by the pressure of the rollers before the pulp box, the pressure and temperature of the chamber, and the thickness of the bundle of filaments. As an alternative, the filaments can be air-textured by passing them through a nozzle by means of an air flow in the form of a jet.

The crimped fibers are then preferably cured to reduce stresses that may be present after the stretching and crimping processes, and they should then be dried. Curing and drying can take place at the same time, typically by feeding the filament bundles from the pulp box, e.g. via an assembly line, through a hot air oven. The temperature in the oven will depend on the composition of the two-component fibres, but must obviously be well below the melting point of the coating component.

The cured and dried filament bundles are then taken to a cutting machine, where the fibers are cut to the desired length. Cutting is typically carried out by passing the fibers over a wheel containing radially arranged knives. The fibers are pressed against the knives by pressure from rollers and are thus cut to the desired length which is equal to the distance between the knives. As explained above, the fibers according to the invention are cut so that they are relatively long, i.e. 3-24 mm, typically 5-20 mm, preferably 6-18 mm, with particularly preferred lengths of 6 mm and approx. 12 mm.

As indicated above, the long thermosetting two-component fiber according to the invention is useful for the production of fluff, i.e. the fluffy, fibrous material used as an absorbent core in the production of hygienic absorbent products such as disposable diapers, sanitary napkins, incontinence products for adults, etc. The use of two-component -the fiber in the manufacture of fluff leads to absorbent materials with superior properties, including, as explained above, improved strength and dimensional stability and more efficient use of the superabsorbent polymer, which makes it possible to manufacture thinner and lighter products and/or products with improved absorption capacity.

A significant part of the downy pulp used for the production of absorbent products typically consists of cellulose pulp fibres. As indicated above, the fluffy mass may also contain additional fibers, e.g. thermosetting synthetic fibers. The cellulose fibers and the synthetic fibers are typically mixed together in a pulp factory and then formed into a so-called blend sheet, which is rolled up into a roll and transported to a converting factory, where the actual production of fluff and absorbent products takes place place. The mixed sheet is formed by a "wet-laid" process, where a wet mixture containing cellulose fibers and synthetic fibers is formed into a sheet which is then conveyed via a conveyor belt to a drying device, typically an oven, where it is dried. Fluff mixtures of fibers can also be produced by a dry process where synthetic fibers from a bale are treated with pulp fibers in the conversion plant. The wet process which produces the mixed sheet is preferred, however, because the mixed sheet can be fed in the form of a roll directly to a hammer mill in the converting plant, which makes the converting process less complicated.

The absorbent material containing the long thermosetting two-component fibers as described above can be prepared as follows: - the two-component fibers and the non-two-component fibers are subjected to mixing through dispersion in water, in a downy pulp production process to obtain a downy pulp mixture where the two-component fibers are distributed over a large set

random and homogeneous manner,

- the wet mixture of two-component and non-two-component fibers is formed into a mixed sheet,

- the mixed sheet is dried and wound up into a roll,

- the dried fluffy mass is fiberized,

- the fluffy mass is formed into a mat,

- a superabsorbent polymer is possibly incorporated into the downy mat (fluff mat), and - the low-melting coating component of the two-component fibers in the material is heat-bonded.

The non-two-component fibers in the fluffy material may comprise a number of different types of natural and/or synthetic fibers according to the particular absorbent material to be produced. Natural cellulose fibers for use in the production of the downy material will typically include bleached grades of CTMP (chemical-thermomechanical pulp), sulphite pulp or kraft pulp.

The weight ratio between the two-component fibers and the non-two-component fibers in the fluffy material (fluff) is preferably in the range from 1:99 to 80:20. It is necessary that the fluff material contains a certain minimum of the two-component fibers so that the improved properties due to the supporting structure of the thermoset two-component fibers can be achieved. Thus, a two-component fiber content of approx. 1% is considered a minimum. On the other hand, the fibers according to the invention do not necessarily have to make up a large proportion of the fluff material. Indeed, one of the advantages of these fibers is that they can be used in a reduced amount compared to the amount typically used in products comprising other currently available thermosetting synthetic fibers. The weight ratio between the two-component fibers and the non-two-component fibers in the fluff material will therefore typically be from 3:97 to 50:50, preferably from 5:95 to 20:80, preferably from 5:95 to 15:85, and especially from 5:95 to 8:92.

The two-component fibers which have been made permanently substantially hydrophilic can easily be distributed in a random and substantially homogeneous manner in the wet downy mass as explained above.

It is possible that during the wet process where the fluffy mass is mixed, a certain amount of the surfactant can in certain cases be removed from the surface of the synthetic two-component fibres. However, it is assumed that this will not lead to a permanent reduction in the hydrophilic properties of the fibres, as it is assumed that the surfactant which is also present in the interior of the fibres' coating component, will later migrate outwards to the surface of the fibers within a short time, typically within 24 h, thereby restoring the fibres' hydrophilic properties.

The wet downy mass is then transferred to a mesh to form a composite sheet which is fed to a drying apparatus, typically an oven, and dried using a temperature significantly lower than the melting point of the coating component of the bicomponent fibers. The mix sheet is typically dried to a water content of 6-9%. The mixing sheet, which typically weighs 550-

750 g/m<2>, and more typically approx. 650 g/m<2>, is then rolled up, and the roll is then normally transported to the conversion plant where the remaining steps in the production of the absorbent material typically take place.

At the conversion plant, the fluffy mass from the roll is typically taken to a hammer mill (as shown in Fig. 4), e.g. via a pair of feed rollers, where the fluffy mass is fibred. Fibering can, however, also be carried out by other methods, e.g. using a spike mill, sawtooth mill or disc grinder. The hammer mill's housing encloses a series of hammers attached to a rotor. The rotor typically has a diameter of e.g. 800 mm, and typically rotates at a speed of e.g. 3000 rpm per my. The hammer mill is typically driven by a motor with an output of e.g. 100 kW. Fibration is achieved as the fibers in the fluffy mass are ejected through the mesh holes in the hammer mill. The size of the mesh holes depends on the type of fluff produced, but they will typically be 10-18 mm in diameter. The two-component fibers should have a length that is compatible with the size of the mesh holes, so that the fibers will survive the fiberization in the hammer mill largely intact. This means that the fibers should not be significantly longer than the diameter of the net holes.

The fibrous fluff is then formed into a fluff mat in a fluff mat forming hood by suction onto a wire cloth, typically followed by passage through a series of condensing or embossing rolls. The mat is preferably compressed (ie either condensed or embossed), but it can also be non-compressed depending on how the absorbent material is to be used. Compression of the mat can alternatively take place either before or after thermosetting.

Before thermosetting, a superabsorbent polymer in the form of a powder or small particles is often incorporated into the material, typically by spraying it into the fluff mat from a nozzle arranged in the fluff mat's forming cap. The purpose of using a super absorbent polymer is to achieve a reduction in the weight and size of the absorbent product, as the amount of fluff in the product can be reduced. The type of superabsorbent polymer used is not critical, but it is typically a chemically cross-linked polyacrylic acid salt, preferably a sodium salt or sodium ammonium salt. Such super absorbent materials are typically able to absorb approx. 60 times their own weight of urine, blood or other body fluids, or approx. 200 times their own weight in pure water. They also have the further advantage that they form a gel upon wetting, which means that the absorbent product can more effectively hold the absorbed liquid under pressure. As indicated above, the superabsorbent polymer is fixed in the desired position in the absorbent material due to the stable matrix structure formed by the two-component fibers by thermosetting. A more efficient use of the superabsorbent polymer is thus achieved, and aggregations of the superabsorbent material which can lead to barriers caused by the gel formed by wetting and swelling are avoided.

One gram of super absorbent polymer can typically replace approx.

5 grams of pulp fiber (e.g. cellulose fiber) in the absorbent material. The superabsorbent polymer is typically incorporated in an amount of 10-70%, preferably 12-40%, preferably 12-20% and especially approx. 15%, calculated on the weight of the material.

After the incorporation of the superabsorbent polymer, the mat is thermoset, e.g. using an oven with air-through, infrared heating or ultrasonic fixing, so that the low-melting component of the bicomponent fibers melts and fuses with other bicomponent fibers and at least some of the non-bicomponent fibers at the same time as the high-melting component of the two-component fibers remains largely intact to form a supporting three-dimensional matrix in the absorbent material (as shown in Fig. 3). Besides giving the absorbent material the improved properties already indicated, this matrix structure also makes it possible to thermoform the absorbent products, e.g. to obtain channels for liquid distribution or to give the products an anatomical shape.

The thermoset absorbent material is then typically formed into units suitable for use in the production of hygienic absorbent products such as disposable nappies, sanitary pads and incontinence products for adults, e.g. when cutting with a water jet. Alternatively, the absorbent material can be formed into such individual units before thermosetting. The remaining material (cuts) can then be returned to the hammer mill to be used again in the production of fluff.

The invention will now be described more fully with reference to the accompanying drawings. Fig. 1 shows two-component fibers where the components are arranged in a concentric (a) and an acentric (b) design. Fig. 2 shows the long two-component fibers and the other fibers in the fluffy mass before thermosetting. Fig. 3 shows the matrix structure formed by the two-component fibers after thermosetting. Fig. 4 shows the hammer mill and the equipment for producing the absorbent material. Fig. 1a shows a cross-section of a two-component fiber 8 with a concentric design. A core component 10 is surrounded by a coating component 12 with a largely uniform thickness, which leads to a two-component fiber where the core component 10 is largely centrally arranged. Fig. 1b shows a cross-section of a two-component fiber 14 with an acentric design. A core component 16 is largely surrounded by a coating component 18 with a varying thickness, which leads to a two-component fiber where the core component 16 is not centrally arranged. Fig. 2 shows the structure of the fluffy mass before thermosetting. Bicomponent fibers 20 according to the invention, comprising a low-melting coating component and a high-melting core component, are arranged in a largely random and homogeneous manner among non-bicomponent fibers 22 in the fluffy mass. Fig. 3 shows the same structure as illustrated in fig. 2 after thermosetting. The covering component of the two-component fibers has been fused by the thermosetting process, whereby the intact core components are fused together 24, so as to form a supporting, three-dimensional base mass. The non-two-component fibers 22 are randomly arranged in the voids delimited by the two-component fibers. Some of the non-bicomponent fibers 22 have been fused 26 with the bicomponent fibers.

In fig. 4, a downy mass 30 from a roll 32 is moistened with water that is sprayed on from a nozzle 34 as the mass is fed to a hammer mill 36. The moistened downy mass is fed into the hammer mill 36 via feed rollers 38. The downy mass 30 comprises a mixture of two components - the fibers according to the invention and other non-two-component fibers. The hammer mill 36 includes a hammer mill housing 40, primary air intake 42 and a secondary air intake 44, hammers 46 attached to a rotor 48, a screen 50 and an outlet 52 for fibrous material 54. A fan 56 carries the fibrous material 54 to a fluff mat forming cap 62 via an exhaust outlet 60. A superabsorbent polymer powder is distributed in the fluff mat 63 via a nozzle 61. The fluff mat 63 is led from a wire cloth 64 through condensation or embossing rollers 66 to another wire cloth 72 where the two-component fibers are thermo-fixed by heat treatment in an oven with through-ventilation 68 where warm air is drawn through the material by means of a suction box 70. A conversion machine 74 is used to produce hygienic absorbent products from the thermoset material.

The roll with the fluffy mass (fluff pulp) 32 comprising, as indicated above, a dried mixture of the two-component fibers according to the invention and non-two-component fibers is produced in a pulp plant and transported to a conversion plant where the process as shown in fig. 4 takes place. Before processing in the hammer mill, the fluffy mass is moistened by a water shower to eliminate the build-up of static electricity. The roll 32 with downy pulp, as it is obtained from the pulp plant, typically has a diameter of e.g. 1000 mm, a width of e.g. 500 mm and a moisture content of 6-9%, and the weight of the sheet is typically approx.

650 g/m<2>. The fluffy mass is fiberized in the hammer mill 36, where the rotating hammers 46 eject the fluffy mass through the holes in the mesh 50. The rotor 48 which holds the hammers 46 typically has a diameter of e.g. 800 mm and rotates at a speed of e.g. 3000 rpm, driven by a motor that has an effect of e.g. 100 kW. The netting 50, which is made from a metal plate with a thickness of approx. 3 mm, contains holes with a diameter of 10-18 mm. The length of the two-component fibers in the fluffy mass 30 is not significantly greater than the diameter of the holes in the mesh 50, so that the two-component fibers as well as the shorter non-two-component fibers can pass through the holes in the mesh 50 largely intact. The fibrous material 54 is then guided by the fan 56 through the exhaust outlet 60 to the forming cap 62 for the fluff mat, where a fluff mat 63 is formed by sucking the fibrous material 54 onto a mesh cloth 64. A superabsorbent polymer powder is typically sprayed from a nozzle 61 when half of the fluff mat 63 is formed, so that the superabsorbent polymer powder lies mostly in the center of the fluff mat 63. The fluff mat 63 typically passes through a series of rollers 66 where the mat 63 is condensed

or embossed before the thermosetting process. The mat 63 is then passed via the second wire cloth 72 past the blow-through oven 68, which thermosets the material to thus produce the supporting structure formed by the core component of the two-component fibers as shown in fig. 3. The thermoset material is then taken to the converting machine 74, where the production of hygienic absorbent products such as diapers takes place.

The invention shall be further illustrated by the following examples.

EXAMPLE 1

Preparation of a permanently hydrophilic, thermosetting, synthetic two-component fiber

Production of the fiber consisted of the following steps:

- incorporating a surface-active agent in the cover component of polyethylene, - subjecting the two components of the fiber to a conventional cover-and-core type melt spinning, which led to a

pre-spun (as-spun) bundle of filaments,

- stretching of the finished spun bundle of filaments,

- ripple of the stretched bundle of filaments,

- hardening and drying of the stretched bundle of filaments, and

- cutting the fibers.

The covering component of the two-component fiber consisted of polyethylene (LLDPE - linear low-density polyethylene, octene-based) with a melting point of 125°C and a density of 0.940 g/cm<3>, while the core component consisted of isotactic polypropylene with a melting point of 160°C. A surfactant was incorporated into the polyethylene component prior to spinning by mixing it into the molten polyethylene to thus make the two-component fibers permanently hydrophilic, hydrophilicity being defined as a sink time in water of no more than 5 s. The surfactant (Atmer® 685 from ICI, a proprietary nonionic surfactant blend) was incorporated in an amount of 1%, based on the total weight of the bicomponent fibers. This corresponded to 2% of the weight of the polyethylene component, as the ratio between polyethylene and polypropylene in the two-component fibers was 50/50. Atmer® 685 is a blend comprising 20% surfactant and 805 polyethylene, with an HLB (hydrophilic-lipophilic balance) value of 5.6 and a viscosity at 25°C of 170 mPa#s.

The polyethylene component was extruded at a temperature of 245°C and a pressure of 35 bar, while the polypropylene component was extruded at a temperature of 320°C and a pressure of 55 bar. The two components were then subjected to conventional cover-and-core melt spinning using a spinning speed of 820 m/min, resulting in a "ready-spun" bundle of two-component filaments.

Separate drawing of the filaments was carried out in a two-stage drawing operation using a combination of hot rollers and a hot air oven, both of which had a temperature of 110°C, with a draw ratio of 3.6:1. The drawn filaments were then crimped in a pulp box crimper. The filaments were cured in an oven at a temperature of 115°C to reduce shrinkage of the fiber during the production of absorbent material and also to achieve a reduction in the water content of the fiber (to 5-10%) and then cut.

The finished two-component fibers had a length of approx. 12 mm, a fineness of 1.7-2.2 dtex and 2-4 ripples per cm.

EXAMPLE 2

Preparation of an absorbent material using CTMP fibers and long hydrophilic thermoset bicomponent synthetic fibers.

The production of the absorbent material comprised the following steps: - mixing of CTMP fibers and the two-component fibers according to the invention during the wet step in a process for the production of fluff pulp,

- drying the fluffy mass,

- fibering of the fluffy mass,

- formation of fluff into a fluff cake and

- thermosetting of the low-melting coating component of the two-component fibres.

In a laboratory hydropulper (British disintegrator) synthetic bicomponent fibers (polypropylene core/polyethylene cover) were mixed with downy pulp CTMP fibers (chemical thermomechanical pulp) in a ratio of 6%:94% (3 g of bicomponent fibers , 47 g CTMP fibers). The two-component fibers had a cut length of 12 mm, a fineness of 1.7-2.2 dtex and 2-4 ripples per cm and was produced as in example 1. The CTMP fibers had a length of approx. 1.8 mm and a thickness of 10-70 jum (average 30 + 10 /xm) . CTMP fibers are produced in a combined chemical and mechanical refining process (as opposed to other pulp fibers which are subjected only to chemical treatment. The two-component fibers which included a surfactant incorporated into the polyethylene coating component as described in Example 1 were hydrophilic and was therefore easily dispersed in the wet fluffy mass.

Drying of the fluffy pulp was carried out in a dryer at a temperature of 60°C, which was well below the melting point of the low-melting component of the bicomponent fibers for a period of 4 h. The dried fluffy pulp (water content 6-9%) weighed 750 g/m<2>. To eliminate static build-up, the dried downy mass was conditioned overnight at 50% relative humidity and a temperature of 23°C.

Fibration was carried out in a laboratory hammer mill (type H-01 Laboratory Defibrator, Kamas Industri AB, Sweden) with a

1.12 kW motor, with hammers attached to a rotor with a diameter of 220 mm and which rotated at a speed of approx. 4500 rpm and with mesh holes with a diameter of 12 mm in a 2 mm thick metal plate. The fluffy material was fed to the hammer mill at a rate of 3.5 g/s. The bicomponent and CTMP fibers, none of which were more than 12 mm long, could both pass large

put intact through the mesh holes in the hammer mill. The fiberization process required an energy consumption of 117 MJ/ton for the mixture of CTMP + 6% two-component fibers, while fiberization of CTMP fluff alone required 98 MJ/ton.

The fibrous mixture was then formed into a fluff cake using standard laboratory pad forming equipment.

The fluffy material was then thermoset by processing in a laboratory oven with hot air at a temperature in the range of 110-130°C (as measured from the air flow immediately after passage through the sample) for a period of 5 s. During the thermosetting process, the low-melting coating component of the bicomponent fibers melted and fused with other bicomponent fibers and some of the CTMP fibers, while the high-melting component of the bicomponent fibers remained intact. The high-melting component of the two-component fibers formed a supportive three-dimensional matrix in the absorbent material and gave it improved properties in terms of improved pad integrity (network strength) and shape retention. The results of pad integrity measurements are shown in Table 1. The test pad formed in a SCAN-C 33 standard specimen forming apparatus weighed 1 g and had a diameter of 50 mm. The test was carried out with an Instron® tensile tester with a PFI measuring device.

EXAMPLE 3

Various permanently hydrophilic, thermosetting synthetic bicomponent fibers were prepared using substantially the same method as in Example 1. The core component of the fibers consisted of polypropylene as described in Example 1, and the weight ratio of the coating/core components of the fibers was 50:50 . The surfactant was the same as that used in example 1 and was used in the same amount of 1% calculated on the total weight of the two-component fibers. The other properties of the fibers were as follows:

EXAMPLE 4

Laboratory tests on test cushions comprising different synthetic two-component fibres

Samples of fluffy material (fluff) were prepared by largely following the procedure according to example 2, the fibers described in example 3 being used as the synthetic two-component fibers. Fluff samples were prepared comprising 94% by weight Scandinavian spruce CTMP pulp and 6% by weight of the respective synthetic fibers. Also, samples containing 3%, 4.5%, 9% and 12% (by weight) of the synthetic fiber were prepared with fibers 1 and 2. As a reference sample, fluff samples were prepared using 100% CTMP pulp.

Blend sheets were produced by first mixing CTMP fibers and the synthetic fibers in water in a British disintegrator as in example 2. The blend sheets were then wet-pressed to a constant thickness (bulk weight = 1.5 cm<3>/g) and dried on a tumble dryer at a temperature of 60°C. There were no difficulties in the production of the composite sheets even with the longest synthetic fibres. The blend sheets were then fiberized in a Kanas H-101 hammer mill as in Example 2 using a 12 mm mesh and a rotational speed of 4500 rpm.

The knot content of the fluffy pulp was determined using a SCAN-C 38 fiber knot tester. The longest fibers (Sample No. 3) tended to form bundles in the fiber knot tester, so the test could not be completed in this case. It was found that the fiber knot content of downy pulp containing 6% synthetic fibers with a length of 6 mm (Samples No. 1 and 4) was only 1%, while the fiber knot content of downy pulp containing 6% synthetic fibers with a length of 12 mm (samples no. 2 and 5) was somewhat higher, 4% and 7% respectively.

Test pads weighing 1 g were formed using a SCAN pad forming apparatus.

Thermosetting was carried out at a temperature of 170°C, as this temperature was found to be suitable in preliminary trials. Heating times of 1, 2 and 4 s were initially tested. The heating time of ls gave the best overall result, and this time was used in the final tests.

Pad integrity of the test pads was measured as described in example 2. The results of these measurements are given in table 2, where the values for network strength are mean values based on 10 samples.

It can be seen from Table 2 that the dry network strength increased significantly after thermosetting as a result of the incorporation of the synthetic two-component fibers according to the invention. Samples 1 and 2 tended to perform somewhat better than the others in this respect. A comparison of the results for sample 1 (6%) with the results for sample 4 shows that crimped fibers are better than non-crimped fibers. The wet network strength of the test pads was also increased by the incorporation of the synthetic fibers, but the increase was not as great as for the dry network strength. Samples 1 and 2 tended to provide an improvement in the wet network strength even before thermosetting.

It was thus shown that the incorporation of relatively small amounts of the synthetic two-component fibers according to the invention provides a significant increase in the strength of the absorbent pads after thermosetting compared to similar pads without the synthetic fibers.

EXAMPLE 5

Synthetic two-component fibers according to the invention were produced as fibers 1 and 2 according to example 3, with the exception that they had a fineness of 1.7 dtex. The fibers were used to produce test pads where the cellulose fibers consisted of either Scandinavian spruce CTMP pulp (fluff quality) or bleached, untreated Scandinavian kraft pulp (Storå Fluff UD 14320) using the same method as in example 4. Reference samples containing either 100 % CTMP or 100% kraft pulp was also prepared.

The network strength of the test pads was measured as described above. The results are shown in Table 3, where the values for network strength are averages based on 10 samples.

The dry network strength of the force test pads was higher than that of the CTMP samples before thermosetting. However, the values were almost the same after thermosetting. The network strength after thermosetting was significantly increased by the incorporation of even small amounts of the synthetic fibers and was almost doubled by the addition of 6% synthetic fibers, compared to the reference test pads that included only CTMP or kraft pulp fibers. The wet network strength of the kraft test pads was somewhat higher than that of the CTMP test pads, both before and after thermosetting. Both the 12 mm and 6 mm synthetic fibers provided an improvement in wet network strength for both CTMP and kraft pulp pads after thermosetting. The difference in wet strength between cushions that have synthetic fiber amounts of between 3 and 9% was relatively small in all cases.

By comparing the results of the network strength measurements for the CTMP pads in this example with the results from samples 1 and 2 in Example 4 above, it can be seen that a somewhat higher network strength was achieved in most cases using the slightly thicker synthetic fibers in Example 4, which had a fineness of 2.2 dtex.

Claims (16)

1. Thermosetting, hydrophilic synthetic two-component fiber for use when mixing a fluffy mass (fluff pulp), comprising an inner core component and an outer coating component, where - the core component comprises a polyalkene or a polyester, - the coating component comprises a polyalkene, and - the core component has a higher melting point than overcoating the component, characterized by the fact that the fiber is permanently largely hydrophilic due to incorporation in the coating component of from 0.1-5%, calculated on the total weight of the fiber, of a surface-active agent, e.g. a fatty acid ester of glycerol, a fatty acid amide, a polyglycol ester, a polyethoxylated amide, another non-ionic surfactant, a cationic surfactant or a mixture of the above and/or other compounds normally used as emulsifiers, surfactants or detergents, and that the fiber has a length of 3-24 mm. 2. Synthetic two-component fiber as stated in claim 1, characterized in that it has a length of 5-20 mm, preferably 6-18 mm, especially approximately 6 mm or approximately 12 mm. 3. Synthetic two-component fiber as stated in claim 1 or 2, characterized in that the surface-active agent has been incorporated into the coating component in an amount of 0.5-2% calculated on the total weight of the fiber. 4. Synthetic two-component fiber as specified in one of claims 1-3, characterized in that the coating component comprises an (ethyl vinyl acetate) or (ethylene acrylic acid) copolymer, or another hydrophilic copolymer or hydrophilic polymer, in particular where the fiber preferably comprises 0 .1-5%, especially 0.5-2% vinyl acetate or acrylic acid calculated on the total weight of the fiber. 5. Synthetic two-component fiber as specified in one of claims 1-4, characterized in that the melting point of the core component is at least 150°C and the melting point of the covering component is 140°C or lower, or that the melting point of the core component is at least 210°C and the melting point for the coating component is 170°C or lower. 6. Synthetic two-component fiber as stated in one of claims 1-5, characterized in that the covering component comprises a polyalkene, e.g. high density polyethylene, low density polyethylene, linear low density polyethylene, polypropylene, poly(1-butene) or copolymers or mixtures thereof, and the core component comprises a high melting polyalkene, e.g. polypropylene or poly(4-methyl-1-pentene), or a polyester, e.g. poly(ethylene terephthalate), poly(butylene terephthalate) or poly(1,4-cyclohexylene dimethyl terephthalate), or copolymers or mixtures thereof. 7. Synthetic two-component fiber as stated in one of claims 1-6, characterized in that it has a fineness of 1-7 dtex, typically 1.5-5 dtex, preferably 1.7-3.3 dtex and preferably 1, 7-2.2 dtex. 8. Synthetic two-component fiber as stated in one of claims 1-7, characterized in that it has been textured to a level of 0-10 ripples per cm, preferably 0-4 ripples per cm. 9. Process for the production of a thermosetting, hydrophilic synthetic two-component fiber of the cover-and-core type for use when mixing down-like pulp, where the fibers have a cover component comprising a polyalkene and a core component comprising a polyalkene or a polyester and which has a higher melting point than the coating component, characterized by melting of the constituents of the core and coating components, - incorporation of from 0.1-5%, calculated on the total weight of the fiber, of a surfactant, e.g. a fatty acid ester of glycerol, a fatty acid amide, a polyglycol ester, a polyethoxylated amide, another non-ionic surfactant, a cationic surfactant or a mixture of the above and/or other compounds which are normally used as emulsifiers, surfactants or detergents in the coating component, - spinning the low-melting coating component and the high-melting core component into a spun bundle of two-component filaments, - stretching the bundle of filaments, - preferably crimping the fibers, - drying and fixing the fibers, and - cutting the fibers to a length of 3-24 mm. 10. Method as stated in claim 19, characterized in that the fibers are cut to a length of 5-20 mm, preferably 6-18 mm, especially approximately 6 mm or approximately 12 mm. 11. Method as stated in claim 9 or 10, characterized in that the surfactant is incorporated into the coating component in an amount of 0.5-2%, calculated on the total weight of the fiber. 12. Method as stated in claims 9-11, characterized in that the fibers' coating component comprises an (ethyl vinyl acetate) or (ethylene acrylic acid) copolymer or another hydrophilic copolymer or polymer, the fiber preferably comprising 0.1-5%, and in particular 0.5-2% vinyl acetate or acrylic acid calculated on the total weight of the fiber. 13. Method as stated in one of claims 9-12, characterized in that the melting point of the core component is at least 150°C and the melting point of the coating component is 140°C or lower, or that the melting point of the core component is at least 210°C and the melting point of the coating component is 170°C or lower. 14. Method as stated in one of claims 9-13, characterized in that the coating component comprises a polyalkene, e.g. high-density polyethylene, low-density polyethylene, linear low-density polyethylene, polypropylene, poly(1-butene) or copolymers or mixtures thereof, and where the core component comprises a high-melting polyalkene, e.g. polypropylene or poly(4-methyl-1-pentene) or a polyester, e.g. poly(ethylene terephthalate), poly(butylene terephthalate) or poly(1,4-cyclohexylene dimethyl terephthalate) or copolymers or mixtures thereof. 15. Method as set forth in one of claims 9-14, characterized in that the bundle of filaments is stretched by using a separate (off-line) stretching process, in particular by using a stretching ratio from 2.5:1 to 4.5:1 and preferably from 3.0:1 to 4.0:1. 16. Method as stated in one of claims 9-15, characterized in that the fibers are stretched to a fineness of 1-7 dtex, typically 1.5-5 dtex, preferably 1.7-3.3 dtex, and preferably 1.7 -2.2 dtex, and preferably textured to a level of 0-10 ripples per cm, preferably 0-4 ripples per cm.